Hi Mike,

You've clearly got a reasonable design going and have done a decent amount of reading up. That's probably why you haven't got a lot of replies making better suggestions!!

The first thing I'd say is go for closed loop steppers rather than open loop. The cost increase is minimal but should prevent lost steps. I'm not even sure the drivers you linked have the (not perfect but better than nothing) stall detection. If you can get AC powered drives it also saves you a lot of PSU hassle.

I'm not overly keen on the lack of central Y axis support but give it a go and as you say you can always use braces or add them in later.

How are you planning to level the two sides of the Y axis and flatten the steel surfaces?

I'd also recommend an extra plate on top of the Y carriages to separate the linear motion components from the gantry itself further. This gives more adjustability to square the two axis to one another.

Keep up the good work!
Andy